A High-Accuracy, Low-Cost Localization System for Wireless Sensor Networks
Radu Stoleru, Tian He, John A. Stankovic, David Luebke
University of Virginia
Department of Computer Science
Overview
• Problem Statement
• State of Art
• Spotlight: THE System
• System Implementation
• Performance Evaluation
• Conclusions
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
Problem Statement
• Localization:
– Find the position of sensor nodes
– Research Challenge:
• Current limitations:
– Sophisticated Hardware: timesynch with satellites (GPS)
– Short Range, Highly Directional (ultrasound)
– Reduced Cost Effectiveness: hefty price for a 1-time event
– Large Form Factor (any solution that requires additional hardware)
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
High Location Accuracy ≡ High Cost
How to solve this, practically?
State of Art
• Many algorithmic solutions, few evaluated in realistic environments
• Solutions that use ranging:– MIT Cricket (Priyantha et at., 2000)
– UCLA AHLoS (Savvides et al., 2000)
– Microsoft RADAR (Bahl et al., 2000)
– UWashington SpotON (Hightower et al., 2000)
– GPS
• Range-free solutions:– USC Centroid (Bulusu et al., 2000)
– MIT Amorphous Computing (Nagpal et al., 2003)
– Virginia APIT (He et al., 2003)
– Rutgers DV-Hop (Niculescu et al., 2003)
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System Design
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• Main Idea: – Generate well-controlled (spatial/temporal) events in the sensor field
– A node generates a timestamp when it detects an event
– Using the above, obtain spatial information (i.e., location) for a node
• Scenario:– Nodes deployed from UAV
(Spotlight device)
– Nodes self-organize, time-synchronize
– The UAV generates light events
– Sensor nodes detect the events and report the timestamps
– The Spotlight device computes the location of the sensor nodes
System Design
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• Assumptions:– Line of Sight
– Position and orientation for the Spotlight device
• GPS/INS (Inertial Navigation System) – cm positioning, arcsec attitude
– Map of deployment area:• LIDAR (Light Detection
and Ranging) 10s centimeter accuracy
• SAR (Synthetic Aperture Radar) 10s centimeter accuracy.
– Powerful Spotlight device• >1000meters
System Design
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
Event DetectionFunction D(e)
Event DistributionFunction E(t)
Localization Function L(T)
e(t)
Ti = { ti1 ti2 ….}
Sensor Node i Spotlight Device
pi(x,y,z)
Generates Events
Timestamp Events
Report Timestamps
Compute Location
Report Location
Spotlight System Architecture
Event Distribution Function: the core of Spotlight We propose 3 Functions
System Design
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
• Event Distribution Functions:– Point Scan
– Area Cover
(0,0) (0,l)
αd
N2
3(b)3(a)
N1 Nj
Ey(tj)
Ex(ti)
0000 0001 0010 0011
0100 0101 0110 0111
1000 1001 1010 1011
1100 1101 1110 1111
t = 2
0000 0001 0010 0011
0100 0101 0110 0111
1000 1001 1010 1011
1100 1101 1110 1111
t = 0
0000 0001 0010 0011
0100 0101 0110 0111
1000 1001 1010 1011
1100 1101 1110 1111
t = 3
0000 0001 0010 0011
0100 0101 0110 0111
1000 1001 1010 1011
1100 1101 1110 1111
t = 1
– Line Scan
0000 0001 0010 0011
0100 0101 0110 0111
1000 1001 1010 1011
1100 1101 1110 1111
System Design
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
• Execution Cost assuming:– All nodes in a square area, with length D
– N events / unit time generated by the Spotlight device
– r is tolerable localization error
• Event Overhead / Unit Time Spotlight Device Power
NrD /)/( 22 NrD /)/2( NDlog r /
D2logrD/22D2D2Event Overhead
logrD21# Time Stamps
logrD21# Detections
Localization Time
Area CoverLine ScanPoint ScanCriterion
System Implementation
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
• Spotlight System
• Spotlight Device:
– Projector
– Laptop
• Mica2 motes
• Short range (10-20m)
• Versatile, it generates:
– Point Scan
– Line Scan
– Area Cover
• Purpose: Investigate Capabilities
System Implementation
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
• Spotlight System
• Spotlight Device:
– Telescope Mount
– Diode Laser
– Laptop
• XSM motes
• Long range (>1000m)
• It generates:
– Point Scan
– Line Scan (recently)
• Demo: later today
Performance Evaluation
• Performance metric:
– Localization Error
– Localization Time
– Localization Range
– Bias in Location Estimation
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
• Parameters Investigated
– EDF (Point & Line Scan, Area Cover)
– Event Size
– Scanning Speed
– Laser Power
– Distance: Spotlight Device, Nodes
Performance Evaluation
0
2
4
6
8
10
12
14
7.0 10.5 14.0 17.5 21.0 24.5
Event Size [cm]
Lo
catio
n e
rro
r [c
m]
87cm/sec
58cm/sec
43cm/sec
35cm/sec
Localization Error vs. Event Size
• Spotlight - Point Scan EDF
0
20
40
60
80
100
120
7.0 10.5 14.0 17.5 21.0 24.5
Event Size [cm]L
oca
liza
tion
Du
ratio
n [s
ec] 87cm/sec
58cm/sec
43cm/sec
35cm/sec
Localization Duration vs. Event Size
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
Performance Evaluation
Localization Error vs. Event Size
• Spotlight - Line Scan EDF
0123456789
10
1.7 3.5 7.0 10.5 14.0 17.5
Event Size [cm]
Lo
catio
n e
rro
r [c
m] 87cm/sec 58cm/sec
43cm/sec 35cm/sec
Localization Duration vs. Event Size
0
5
10
15
20
1.7 3.5 7.0 10.5 14.0 17.5
Event Size [cm]
Lo
caliz
atio
n D
ura
tion
[se
c]
87cm/sec 58cm/sec
43cm/sec 35cm/sec
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
Performance Evaluation
Position Estimation Bias
• Spotlight - Line Scan EDF
-6
-5
-4
-3
-2
-1
0
1
2
3
1.7 3.5 7.0 10.5 14.0 17.5
Event Size [cm]
Ho
rizo
nta
l Bia
s [c
m]
87cm/sec 58cm/sec
43cm/sec 35cm/sec
Position Estimation w/o Bias (ideal)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.7 3.5 7.0 10.5 14.0 17.5
Event Size [cm]L
oca
liza
tion
Err
or
w/o
Bia
s [c
m]
87cm/sec 58cm/sec
43cm/sec 35cm/sec
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
Performance Evaluation
Localization Error vs. Event Size
• Spotlight - Area Scan EDF
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
6 8 10 12
Number of Bits
Lo
catio
n e
rro
r [c
m]
20ms/event 40ms/event 60ms/event
80ms/event 100ms/event
Localization Duration vs. Event Size
012345
6789
10
6 8 10 12
Number of BitsL
oca
liza
tion
Du
ratio
n [s
ec] 20ms/event 40ms/event 60ms/event
80ms/event 100ms/event
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
Performance Evaluation
Localization Error vs. Event Size
• Spotlight - Line Scan EDF
Localization Duration vs. Event Size
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
0
5
10
15
20
25
6 12 18
Event Size [cm]
Lo
caliz
atio
n E
rro
r [c
m] 0.7m/sec
1.4m/sec
3m/sec
0
5
10
15
20
25
30
35
40
0 25 50 75 100 125 150
Event Size [cm]L
oca
liza
tion
Tim
e [m
in] 3m/sec
6m/sec
9m/sec
Performance Evaluation
Localization Range
• Spotlight - Line Scan EDF
950
1000
1050
1100
0 50 100 150 200
Distance [m]
Inte
nsi
ty [A
DC
co
un
t] 35mW
7mW
Detectable Event Sizes
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0
200
400
600
800
1000
1200
0 50 100 150 200
Event Size [cm]
Inte
nsi
ty [A
DC
co
un
t]
35mW
7mW
Future Work
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
• Distributed Spotlight System
• Localization Overhead Reduction
• Dynamic Event Distribution Function
• Stealthiness
• Deployment in Unknown Terrain
• Self-Calibration
• Event Distribution Detection and Reporting
Conclusions
UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.
• Contributions: designed, implemented, evaluated the Spotlight Localization System
• Main ideas:
–
– Well-controlled (spatio-temporal) events in the network. Infer spatial information from temporal knowledge
• Long Effective Range (1000s meters)
• High Accuracy (10s centimeter)
• No hardware changes to existing motes
• Reusable localization system (amortized cost)
Complexity
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Thank you!